1. Field of the Invention
The present invention relates to an antenna device and a radar module used in the millimeter wave range.
2. Description of the Related Art
As for transmission lines for use in the microwave and millimeter-wave ranges, waveguides, coaxial transmission lines, and transmission lines of the type comprising a conductor formed on a dielectric substrate, such as microstrip transmission lines, coplanar transmission lines and slot transmission lines, are widely used. When a transmission line is formed on a dielectric substrate, it is possible to easily connect the transmission line to another electronic component such as an integrated circuit. Taking this advantage, a various kinds of integrated circuits are formed by mounting electronic components on a dielectric substrate.
As for antennas for use in the millimeter-wave range, waveguide horn antennas and microstrip line patch antennas are used.
Microstrip transmission lines, coplanar transmission lines, and slot transmission lines have a rather large transmission loss, and thus they are not suitable for use in circuits which need a low transmission loss. To solve the above problem, the applicant for the present invention has filed a patent in terms of planar dielectric transmission line and an integrated circuit which is disclosed in laid-open Japanese Patent Application No. 8-265007.
When this planar dielectric transmission line is used to form an antenna device for use in for example a millimeter-wave radar installed on a car, the transmission mode is converted to a waveguide mode so as to form a waveguide horn antenna, or is converted to a microstrip line transmission mode via a coplanar transmission mode whereby a signal is supplied to a microstrip line patch antenna. However, the advantages of being low in the transmission loss and small in the size provided by the planar dielectric transmission line are lost because the use of a transmission converter for achieving the transmission mode conversion causes an increase in the total volume of the module, and a loss occurs when an RF signal passes through the transmission converter, which results in a reduction in the antenna efficiency. Another problem is that a complicated assembling process is needed. Furthermore, the repeatability of the characteristics becomes poor. As a result, the total cost increases.